1 /* LibTomCrypt, modular cryptographic library -- Tom St Denis
2 *
3 * LibTomCrypt is a library that provides various cryptographic
4 * algorithms in a highly modular and flexible manner.
5 *
6 * The library is free for all purposes without any express
7 * guarantee it works.
8 *
9 * Tom St Denis, tomstdenis@gmail.com, http://libtomcrypt.com
10 */
11 #include "tomcrypt.h"
12
13 /**
14 @file der_length_integer.c
15 ASN.1 DER, get length of encoding, Tom St Denis
16 */
17
18
19 #ifdef LTC_DER
20 /**
21 Gets length of DER encoding of num
22 @param num The int to get the size of
23 @param outlen [out] The length of the DER encoding for the given integer
24 @return CRYPT_OK if successful
25 */
der_length_integer(void * num,unsigned long * outlen)26 int der_length_integer(void *num, unsigned long *outlen)
27 {
28 unsigned long z, len;
29 int leading_zero;
30
31 LTC_ARGCHK(num != NULL);
32 LTC_ARGCHK(outlen != NULL);
33
34 if (mp_cmp_d(num, 0) != LTC_MP_LT) {
35 /* positive */
36
37 /* we only need a leading zero if the msb of the first byte is one */
38 if ((mp_count_bits(num) & 7) == 0 || mp_iszero(num) == LTC_MP_YES) {
39 leading_zero = 1;
40 } else {
41 leading_zero = 0;
42 }
43
44 /* size for bignum */
45 z = len = leading_zero + mp_unsigned_bin_size(num);
46 } else {
47 /* it's negative */
48 /* find power of 2 that is a multiple of eight and greater than count bits */
49 leading_zero = 0;
50 z = mp_count_bits(num);
51 z = z + (8 - (z & 7));
52 if (((mp_cnt_lsb(num)+1)==mp_count_bits(num)) && ((mp_count_bits(num)&7)==0)) --z;
53 len = z = z >> 3;
54 }
55
56 /* now we need a length */
57 if (z < 128) {
58 /* short form */
59 ++len;
60 } else {
61 /* long form (relies on z != 0), assumes length bytes < 128 */
62 ++len;
63
64 while (z) {
65 ++len;
66 z >>= 8;
67 }
68 }
69
70 /* we need a 0x02 to indicate it's INTEGER */
71 ++len;
72
73 /* return length */
74 *outlen = len;
75 return CRYPT_OK;
76 }
77
78 #endif
79
80 /* $Source: /cvs/libtom/libtomcrypt/src/pk/asn1/der/integer/der_length_integer.c,v $ */
81 /* $Revision: 1.4 $ */
82 /* $Date: 2006/04/22 01:22:55 $ */
83